Spread spectrum is a digital radio frequency signaling technique in which a synchronized code is used by a transmitter and receiver pair to respectively spread and de-spread a transmitted data sequence over a predefined bandwidth. As the name suggests, spread spectrum systems utilize more bandwidth than other conventional signaling techniques such as time-division multiple access (TDMA). However, the improved security and noise rejection attained by spread spectrum systems compensates for the increased bandwidth requirements.
Spread spectrum modulation was originally developed for military use, where secure communications were required. However, due to its unique multipath and interference rejection characteristics, spread spectrum has civilian applications in mobile radio environments. Spread spectrum techniques are especially well suited for applications where a number of independent users need to share a common band of frequencies without the benefit of an external synchronizing mechanism, such as in code-division multiple access (CDMA) cellular radio systems.
CDMA techniques are well known. Some well known CDMA systems employ coherent detection for both directions of the communication path (i.e., base station to mobile and vice versa). Coherent detection (with phase) offers significant advantages over noncoherent detection (without phase). However, for the conventional phase locked loop type of phase synchronization, a stable signal with a high signal-to-noise ratio is required to track the unknown phase. For the reverse link (mobile unit to base station) of CDMA systems, several reasons make such phase tracking impractical. First, the spreaded signal has very low signal-to-noise ratio which cannot be used for tracking the phase of the signal. Second, fast fading makes the signal unstable and shifts the unknown phase. Thus, a very fast tracking loop is required to maintain a good estimation of the phase. In addition, the bursty nature of voice activity disrupts the signal. Further, excessive power consumption precludes transmitting a pilot signal from the mobile. Moreover, a rake receiver is typically used to combine multipath fading. If a plurality of multipath signals are used for the demodulation, the signal-to-noise ratio per path to achieve the desired performance will be significantly reduced. However, each path experiences independent phase shift. Thus, the phase tracking loop has to work with a very low signal-to-noise ratio for each path. Finally, any phase tracking circuitry built to overcome the above difficulties, will be complicated and expensive. Rake receivers comprise multiple data receivers, thus, the complexity of the circuitry required for phase tracking is dependant upon the number of demodulated paths which is typically relatively high.
Because of the difficulties associated with coherent detection, noncoherent detection is typically used in the reverse link of CDMA systems. In order to achieve decoding in many CDMA systems, both an orthogonal code and a convolutional code have to be soft-decision decoded. If orthogonal coding were the outer layer (first step in transmit and last step in receive), its soft decision decoding would be straightforward. However, in order to soft-decision decode the convolutional code, the decoder of the orthogonal code must develop a likelihood for each convolutional code symbol.
Prior art approaches to this issue have certain drawbacks. For instance, prior art approaches do not deliver the signal-to-noise ratios achievable if coherent detection could be used. What is needed therefore, is a method for estimating the unknown phase and strength of a noncoherently detected signal in order to increase the received signal-to-noise ratio of symbols transmitted in code-division multiple access communication systems and, thus, the capacity of such systems.